U.S. patent number 4,583,852 [Application Number 06/481,027] was granted by the patent office on 1986-04-22 for attitude transfer system.
This patent grant is currently assigned to The Perkin-Elmer Corporation. Invention is credited to Lawrence W. Cassidy, Douglas R. Everhart.
United States Patent |
4,583,852 |
Cassidy , et al. |
April 22, 1986 |
Attitude transfer system
Abstract
A system for measuring the relative orientation between two
objects. A transmitter/receiver assembly on one of the objects
transmits a beam of monochromatic light to a Ronchi-type
retro-grating disposed on the other object. The grating reflects
the light back to one or more charge transfer device area arrays in
the transmitter/receiver assembly which provides orientation data
to a computer which determines relative pitch, yaw and roll between
the two objects.
Inventors: |
Cassidy; Lawrence W. (New
Milford, CT), Everhart; Douglas R. (Danbury, CT) |
Assignee: |
The Perkin-Elmer Corporation
(Norwalk, CT)
|
Family
ID: |
23910291 |
Appl.
No.: |
06/481,027 |
Filed: |
March 31, 1983 |
Current U.S.
Class: |
356/139.03;
356/152.2; 356/508; 356/521 |
Current CPC
Class: |
G01B
11/26 (20130101); G01C 25/005 (20130101); G01C
21/24 (20130101); G01C 15/002 (20130101) |
Current International
Class: |
G01B
11/26 (20060101); G01C 25/00 (20060101); G01C
15/00 (20060101); G01C 21/24 (20060101); G01B
011/26 (); G01C 001/00 () |
Field of
Search: |
;356/152,354,356,363
;244/3.13,3.16 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Buczinski; S. C.
Assistant Examiner: Wallace; Linda J.
Attorney, Agent or Firm: Murphy; Thomas P. Grimes; Edwin T.
Wilder; Richard C.
Claims
What is claimed is:
1. A system for determining attitude of a first object relative to
a second object comprising:
first means on said first object for transmitting a beam of
monochromatic light,
lines grating means on said second object disposed in the path of
said beam of monochromatic light for reflecting said beam of
monochromatic light as a diffracted fan of collimated light bundles
back to said first means,
said first means including sensor means having a face disposed in
the path of said reflected bundles of light for determining pitch,
yaw and roll based on the position of said light bundles falling on
said face of said sensor means.
2. A system according to claim 1 wherein said reflected bundles of
light comprise one bundle of zero order and other bundles of
ascending orders.
3. A system according to claim 2 wherein said reflected bundles of
light form a line orthogonal to the lines of said line grating
means.
4. A system according to claim 3 wherein said sensor means
includes,
at least one charge transfer device area array disposed in the path
of said reflected bundles of light for sensing the light intensity
of said zero order and some of the other orders of said bundles of
light.
5. A system according to claim 4 wherein said first object is an
inertial reference platform.
6. A system according to claim 5 wherein said sensor means includes
second means for determining the centroid of selected ones of said
reflected bundles of light falling on said face of said sensor
means.
7. A system according to claim 6 wherein said first means
includes,
optical means for transmitting said beam of monochromatic light and
focussing said reflected bundles of light on said face of said
sensor means.
8. A system according to claim 7 wherein said line grating means
comprises,
a ruled reflection grating of the Ronchi type fixed to said second
object in the path of said monochromatic beam of light.
9. A system according to claim 8 wherein said face of said charge
transfer device area array comprises,
a matrix of light collecting pixel elements arranged in an X-Y
coordinate system.
10. A system according to claim 9 wherein said second means
comprises,
a microprocessor for determining the position of said first order
reflected light bundle relative to the coordinate system of said
matrix and of the angle a line connecting the two extreme bundles
of light falling on said matrix makes with the coordinate system of
said matrix.
Description
BACKGROUND OF THE INVENTION
Precise information of relative attitude between remote objects is
necessary for a variety of reasons. For instance, in a spacecraft
it is often important to determine variances in attitude between
the inertial reference platform and objects such as boom mounted,
steerable antennas, remote sensor platforms and STS pallet-mounted
experiments to name a few. Such attitude information is necessary
so that data received from or provided to such objects may be
corrected or calibrated for relative deviation in attitudes between
the inertial reference platform and the object of interest.
Use of star trackers or gyros to obtain attitude information for
individual objects is often prohibitive in cost as well as space
and weight requirements. Thus, systems which measure attitude
without the necessity that each object include its own attitude
sensing apparatus are highly desirable and, in fact, are in
existence today.
One such system requires two transmitter/receiver assemblies
mounted, e.g., on the inertial reference platform. One
transmitter/receiver measures pitch and yaw through autocollimation
by reflecting a beam of monochromatic light from a mirror mounted
on the remote platform whose attitude is to be measured. Sensors in
the transmitter/receiver provide information of the pitch and yaw.
The second transmitter/receiver is necessary to determine roll of
the remote platform.
A second system ultilizes a single transmitter/receiver. Pitch and
yaw are obtained as above but roll is obtained through the use of
an active source which must be mounted on the remote platform to
direct a beam of polarized light back to the
transmitter/receiver.
The present invention is an improvement over the above described
systems and requires only a single transmitter/receiver on the
inertial reference platform without the necessity of active sources
mounted on the remote platform.
BRIEF SUMMARY OF THE PRESENT INVENTION
The present invention relates to a three axis attitude transfer
system for measuring pitch, yaw and roll of a remote object or
platform relative to a reference platform. More particularly, the
present invention comprises a transmitter/receiver assembly
disposed on the reference platform. The transmitter/receiver
assembly includes means for transmitting a beam of monochromatic
light to a ruled reflection grating on the remote platform. The
grating reflects the beam as a fan of collimated light bundles of
varied intensity. These return light bundles which are dispersed
from the grating lie in a plane which is orthongonal to the rulings
of the grating. The reflected beams fall on a sensor in the
transmitter/receiver assembly. The sensor which may comprise one or
more charge transfer device (CTD) area arrays provides data to a
computer which centroids all detected images in accordance with a
specified algorithm to provide pitch and yaw information from the
location of the zero order beam relative to the coordinates of the
sensor area array and roll information from the angle between the
best fit straight line through the image locations and the sensor
array coordinate system.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic representation of an embodiment of the
present invention;
FIG. 2 is a schematic representation of the sensor arrangement of
the present invention and the line of reflected beams;
FIG. 3 is a graphic representation of a reflected light beam
falling on a portion of an area array useful in understanding the
present invention; and
FIG. 4 is a block diagram illustrating the circuitry for
calculating pitch, yaw and roll in the present invention.
DESCRIPTION
FIG. 1 shows a transmitter/receiver assembly 11. The
transmitter/receiver assembly 11, hereinafter referred to as
transceiver 11, is normally mounted within an inertial reference
platform of the type used in spacecraft for determining position
and attitude in space.
The remote platform whose attitude is to be meaured has a ruled
grating 12 of the Ronchi type fixed thereto. The remote platform is
generally located some distance from the inertial reference
platform. It may be within the spacecraft such as a remote sensor
platform or outside the spacecraft such as an antenna controlled
from the spacecraft by means of a boom. In any event the remote
platform is subject to variations in pitch, yaw and roll relative
to the inertial reference system due to racking, vibrations and the
like to which the spacecraft may be subject.
The transceiver 11 comprises means for transmitting a beam of
monochromatic light to the grating 12 where it is diffracted and
reflected back to the transceiver as a fan of light bundles or
beams of varying intensities and orders.
In particular, the transceiver 11 comprises a laser source 13, a
beamsplitter 14 and a pair of mirrors 15 and 16. The mirror 15 has
a concave surface facing the convex surface of mirror 16. As can be
seen in FIG. 1 mirror 15 has a central opening with mirror 16
disposed with its optical axis coincident with that of mirror 15.
The beamsplitter 14 is positioned relative to laser source 13 and
mirrors 15 and 16 so that a laser beam from the laser source 13
reflects from beamsplitter 14 and mirrors 15 and 16 to be directed
toward grating 12. The optics for transmitting the beam are
conventional and their function may be carried out by other
conventional optics such as a refractive lens system.
The transceiver 11 further comprises a pair of identical charge
transfer device area arrays 17 and 18 disposed in the same plane
one above the other for receiving the fan of light bundles or beams
reflected from line grating 12. The charge transfer device area
arrays 17 and 18 are commercially available devices and are
available, e.g., from the General Electric Company. Each of the
arrays 17 and 18 may, e.g., comprise up to five hundred pixels per
side. The array of pixels is partially shown in the upper left hand
corner of CTD array 18 shown in FIG. 2. Each pixel area is an
individual light sensor and provides an output voltage
representative of the intensity of the light incident thereon.
As seen in FIG. 1 the laser beam shown by solid lines is
transmitted to line grating 12 of the remote platform. The zero
order reflected beam returns to the transceiver and passes through
the optics of mirrors 15 and 16 and beamsplitter 14 to be focused
at point P.sub.1 on CTD area array 17 as shown in FIG. 2.
FIG. 1 also shows one of the diffracted orders of beams in dashed
lines reflected by the line grating 12. This order is reflected at
an angle .theta., with respect to the optical axis so that it is
directed back through the optics to focus as point P.sub.2 on CTD
area array 18 shown in FIG. 2. Of course, the fan of light bundles
reflected from line gratings are numerous some of which are shown
as dark dots in FIG. 2. The fan of light bundles are by definition
orthogonal to the direction of lines on line grating 12. This
relationship remains true even though the remote platform may
deviate in pitch, yaw and roll relative to the inertial reference
platform which contains the transceiver 11. Thus, the angle that
the line of images of the light bundles makes with the Y axis of
the CTD arrays 17 and 18 provides roll information. The position of
the zero order image P.sub.1 with respect to the center of CTD
array 17 provides pitch and yaw information to better than one
arcsecond of accuracy. The two extreme reflected bundles of light
P.sub.1 and P.sub.2 in FIG. 2 provide sufficient information to
obtain roll to less than one arcsecond of accuracy. Points P.sub.1
and P.sub.3 on CTD area array 17 are sufficient to give roll
information to about 8 arcseconds permitting CTD area array 18 to
be eliminated if roll accuracy of 8 arcseconds is adequate. This,
accuracy in roll measurement is a function of the distance between
CTD area arrays 17 and 18 with accuracy increasing as the distance
therebetween is increased. In a practical embodiment each CTD area
array is 0.4 inches square with the overall separation between the
two arrays capable of being varied over a range of several
inches.
The CTD area arrays provide data inputs to a microprocessor, e.g.,
a Motorola MC 68000 which centroids all detected images to derive
pitch and yaw information from the location of point P.sub.1 and
roll information from the angle between the line joining points
P.sub.1 and P.sub.2 and the array coordinate system.
As seen in FIG. 3 each reflected bundle of light overlaps several
pixel areas on the CTD area array 17 and 18. This permits use of a
standard center of mass algorithm programmed into the
microprocessor to determine the centroid of the returning bundle of
light relative to the coordinate system of the CTD area arrays 17
and 18.
As is well known, each pixel area of a CTD area array acts as a
light collector. Thus, when the pixels of a CTD area array are
turned on for its integration period which may last, e.g. a tenth
of a second, photons of the light falling on a pixel area are
converted to electrons and accumulated in each pixel in proportion
to the local light intensity. After the integration period when all
the pixels of a CTD area array are turned off the accumulated
charge is automatically transferred out to the utilization device
which in the present invention is microprocessor 19.
Considering the 3.times.3 pixel matrix of FIG. 3 it is seen that a
bundle of light reflected from line grating 12 may be imaged at a
random location on either CTD 17 or 18 and each image may overlap
up to nine pixels M.sub.1 -M.sub.9. By comparing the light
collected in each of the pixels relative to the others, the
centroid given by coordinates X and Y may be found.
This calculation may be performed in a microprocessor by a single
center of mass algorithm substituting the amount of light collected
by each pixel (and digitally encoded) for mass. Thus, the
algorithm: ##EQU1## where X and Y are the image location
coordinates of points P.sub.1 M.sub.i are the signals integrated
within each sampled pixel, and X.sub.i and Y.sub.i are the
coordinates of each sampled pixel's center.
may be used to determine the centroid of point P.sub.1 which
provides the pitch and yaw information of the remote platform. The
centroids of points P.sub.1 and P.sub.2 which provide roll
information of the remote platform.
In particular to derive the relative pitch and yaw angles the
microprocessor performs the following algorithm using point P.sub.1
centroid coordinates ##EQU2## where,
Y-Y.sub.0 is the distance of the P.sub.1 image centroid from the
center of CTD 17 measured in micrometer units along the Y axis.
X-X.sub.o is the distance of the P.sub.1 image centroid from the
center of CTD 17 measured in micrometer units along the X axis,
F is the effective focal length of the transceiver optical system
(15 and 16) measured in micrometer units.
The location X.sub.o, Y.sub.o is a data base item stored in the
microprocessors memory along with other calibration data including
the value of F.
To obtain the relative role angle the microprocessor implements the
following algorithm ##EQU3## where, X.sub.2 -X.sub.1 is the
separation between the centroid locations of images P.sub.2 and
P.sub.1 measured along the X axis, Y.sub.2 -Y.sub.1 is the
separation between the centroid locations of images P.sub.2 and
P.sub.1 measured along the Y axis
The data collected by CTD area arrays is transferred to
microprocessor 19 at the end of each integration period and may be
updated at rates up to 30 Hz.
During the transfer process photoelectrons generated within each
pixel during the previous integration period are transferred in
bucket brigade fashion to an output preamplifier (20) shown in FIG.
4 on each CTD area array. Here each charge packet is amplified and
is then filtered and encoded into one of 255 digital values (8-bit
quantization) via off chip electronic circuitry (21). The digital
values (typically 9 from image P.sub.1 and 9 from image P.sub.2)
are then passed to the microprocessor (19) for the computation of
pitch, roll and yaw. Along with these digitized signals, the
address of each sampled pixel (two 9-bit words per pixel, one word
denoting the X coordinate of the pixel and one word denoting the Y
coordinate of the pixel) are forwarded to the microprocessor. Time
information in the form of a digital word provided by a clock
circuit (22) completes the information needed for alignment
computation.
The present invention, of course, is not limited to spacecraft but
may find use in other vehicles, e.g., aircraft, land vehicles and
where the remote object or platform is located internally or
externally to the vehicle.
Other modifications of the present invention are possible in light
of the above description which should not be construed as placing
limitation beyond those set forth in the claims which follow:
* * * * *